Caution: Translations of Quran and Ḥadīth may lead to possible misapplications and misinterpretations. This site is intended for students of sacred knowledge that are proficient in comprehending classical Arabic and have a strong foundation in Islamic sciences. Also note that religious injunctions rely on several aspects beyond what one may glean through reading individual aḥādīth.
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السيوطي:٩٤٤٥a

"أَيُّمَا رَجُلٍ مُسْلِمٍ أكْفَرَ رَجُلًا مُسْلِمًا فَإِنْ كَانَ كَافِرًا، وَإلَّا كَانَ هُوَ الكَافِر".  

[د] أبو داود عن ابن عمرو

See similar narrations below:

Collected by Abū Dāwūd
abudawud:4687ʿUthmān b. Abū Shaybah > Jarīr > Fuḍayl b. Ghazwān > Nāfiʿ > Ibn ʿUmar

If any believing man calls another believing man an unbeliever, if he is actually an infidel, it is all right ; if not, he will become an infidel.  

أبو داود:٤٦٨٧حَدَّثَنَا عُثْمَانُ بْنُ أَبِي شَيْبَةَ حَدَّثَنَا جَرِيرٌ عَنْ فُضَيْلِ بْنِ غَزْوَانَ عَنْ نَافِعٍ عَنِ ابْنِ عُمَرَ قَالَ

قَالَ رَسُولُ اللَّهِ ﷺ أَيُّمَا رَجُلٍ مُسْلِمٍ أَكْفَرَ رَجُلاً مُسْلِمًا فَإِنْ كَانَ كَافِرًا وَإِلاَّ كَانَ هُوَ الْكَافِرَ